1. Field of the Invention
The present invention relates to a vinylidene fluoride resin composition and, more particularly, to a vinylidene fluoride resion composition having improved thermal stability and electrical properties. 2. Brief Description of the Prior Art
Vinylidene fluoride resin is generally superior in thermal stability, but it is subject to discoloration to dark brown when extruded under severe conditions at temperatures higher than 200.degree. C., particularly higher than 250.degree. C. This discoloration not only impairs the appearance of the product but also deteriorates the quality of the product due to fluoride ions in defluorination. Where vinylidene fluoride is used as a capacitor film, this discoloration decreases the insulation resistance and life of the capacitor. The cause of such discoloration is not known completely, but the formation of conjugated double bonds resulting from defluorination is considered to be one cause, in view of the fact that discolored resin contains more fluorine ions than intact resin and that carbonyl groups and double bonds are detected in discolored resin by the infrared analysis.
It has been considered that an agent that capures liberated hydrochloric acid would be useful to improve the thermal stability of vinylidene fluoride resin as in vinyl chloride resin. In actual, however, none has been found effective for vinylidene fluoride resin among the existing stabilizers including organotin stabilizers for vinyl chloride resin, metal soaps represented by lead stearate, amine stabilizers represented by aminocrotonic acid ester, and epoxy compounds represented by epoxidized vegetable oil. They sometimes exhibit even adverse effects.
In connection with the improvement of thermal stability of vinylidene fluoride resin, there is disclosed in Japanese Patent Laid-Open No. 149243/1978 the incorporation of phosphinic acid (salt). It improves the thermal stability at the time of processing, but the addition of an inorganic salt deteriorates considerably the electrical properties of molded articles. There is disclosed the blending of an epoxy resin with vinylidene fluoride resin in Japanese Patent Laid-Open No. 100447/1979, U.S. Pat. No. 3976617, and Soviet Pat. Nos. 492530 and 492531. These disclosures relate to improving the adhesion of vinylidene fluoride resin to a metal substrate and to preventing the vinylidene fluoride resin coating from blistering in boiling water, by the aid of epoxy resin blended therein.
In the expectation that epoxy resin would capture hydrofluoric acid, the present inventors studied the thermal stability of vinylidene fluoride resin incorporated with epoxy resin. Contrary to the expectation, the improvement of thermal stability was not achieved and it was found that epoxy resin is poor in compatibility and deleterious to transparency.